This invention relates to direct sequence modulated spread spectrum transmission systems. More specifically, this invention relates to digital circuitry for suppressed clock pulse-duration modulation in direct sequence modulated spread spectrum systems.
As is known in the art, in a spread spectrum transmission system, a signal or operation other than the information being sent is used for broadbanding the transmitted signal. Thus, in a spread spectrum system, the transmitted information is literally spread over a frequency range that is much wider than the minimum bandwidth required to transmit that information. Because of the low-density power spectra of the transmitted signal, and the ability of such systems to operate reliably with signal-to-noise ratios substantially less than unity, spread spectrum transmission systems can be used as secure communication systems in which the transmitted signal in effect is hidden in background noise that results from atmospheric and other natural causes as well as other transmission systems.
One type of modulation that can be employed in a spread spectrum communication is known as "direct sequence modulation." In a digital circuit direct sequence modulated spread spectrum system, the carrier signal is modulated by a digital information signal and by a digital code sequence that exhibits a bit rate that is substantially higher than the rate at which digital information is made available to the system. In many such systems, the digital code sequence is a pseudo-random binary code sequence that is pulse code modulated at a predetermined clock rate. In particular, direct sequence spread spectrum systems often utilize conventional circuit arrangements that are commonly identified as pseudo-noise generators which are responsive to an applied periodic clock signal and produce a pseudo-random binary sequence in a pulse code modulated format. In most cases, the information to be transmitted is combined with the binary code sequence by modulo-2 addition that is effected prior to modulating the system carrier signal. Thus, the direct sequence modulation signal is a pulse code modulated digital signal having a clock frequency equal to the clock frequency of the pseudo-noise generator. When the carrier signal is modulated with the data-embedded pseudo-noise code, the information can readily be recovered by a receiver that demodulates the received signal and correlates the demodulated signal with a reference signal that is synchronized to and is identical to the pseudo-random binary sequence utilized in the spectrum.
Although direct sequence spread spectrum systems of the above-described type exhibit a low-density power spectra (i.e., the power transmitted is relatively low in any narrow frequency band), such a system often does not provide the desired degree of security relative to detection and interception of the coded transmission. In particular, the power spectra of both the direct sequence modulation signal and the modulated carrier signal exhibit a (Sin x/x).sup.2 envelope in which the main lobe of the envelope exhibits a bandwidth (null-to-null) equal to twice the pseudo-noise generator clock rate and each side lobe of the envelope exhibits a bandwidth that is equal to the pseudo-noise generator clock rate. Thus, by utilizing a narrow band receiver that is continuously tuned (swept) to search the frequency spectra, it is possible for unauthorized persons to detect the transmission of a direct sequence modulated signal and to determine the clock rate of the system pseudo-noise generator. In spread spectrum systems of the type intended to "hide" the fact that a signal is being transmitted, detection of the (Sin x/x).sup.2 feature can severely limit the effectiveness of the system. Further, in some applications, detection of the system clock rate by an unauthorized receiver can lead to either interception of the transmitted information or permit the intercepting party to generate a jamming signal at the system clock rate of sufficient power density to temporarily disable the associated navigation or communication system.
A solution to the above-discused drawback has been proposed relative to spread spectrum systems in which the information to be transmitted is supplied to the system in analog signal format. More specifically, in a signal processing technique that is referred to as suppressed clock pulse-duration modulation, an analog input signal (the information to be transmitted) is supplied to a sample-and-hold circuit that is clocked at a rate that is substantially higher than the highest frequency component of the input signal. The output of the sample-and-hold circuit is supplied to one input terminal of a comparator circuit having the second input terminal thereof connected for receiving a signal supplied by a ramp generator that is synchronized with the sample-and-hold circuit. Since the signal comparator changes states when the value of the ramp signal exceeds the signal level stored in the sample-and-hold circuit, the signal supplied by the comparator is a pulse-duration modulated signal, with the length of each pulse ranging between zero and the clock period and being dependent upon the current value of the sampled signal. Since the ramp generator is synchronized to the sample-and-hold circuit, the pulse-duration modulation signal generated by the comparator circuit is synchronous with the clock signal, i.e., one edge of each pulse of the pulse-duration modulation signal is coincident in time with the falling edge of the ramp signal. To eliminate the clock feature from the pulse-duration modulation signal, the output of the comparator circuit is connected to a modulo-2 adder, which also is supplied with a periodic signal that is generated by dividing the clock signal by two. The modulo-2 addition, in effect, subtracts the clock signal from the pulse-duration modulated data stream provided by the comparator circuit thus, embedding the information to be transmitted in a signal that exhibits one-half the circuit sampling rate (clock rate). To restore the pulse-duration modulated signal within the system receiver, the suppressed clock-pulse-duration modulated signal is modulo-2 added with a square wave that is synchronized to and operates at one-half the clock rate of the clock circuit that controls the sample-and-hold circuit and ramp generator of the system transmitter.
The above-described suppressed clock pulse-duration modulation technique is not directly applicable to digital direct sequence modulated spread spectrum systems since the direct sequence modulation signal is a binary code sequence that cannot be supplied to the sample-and-hold circuit in place of the analog input signal. Moreover, satisfactory clock suppression generally cannot be obtained by converting the direct sequence modulation signal to an analog signal and applying that analog signal to a suppressed clock pulse-duration modulation circuit of the above-described type. In particular, time delays that are primarily attributable to the fall-time of the ramp generator and the settling time of conventional digital-to-analog converter circuits cause the comparator circuit to provide a pulse-duration modulation signal that is delayed in time relative to the system clock signal. Because of this time delay, the clock signal is not entirely suppressed when the delayed pulse-duration modulation signal is modulo-2 added with a signal that is synchronized to and operates at one-half the digital-to-analog conversion rate. Although the degree of clock suppression that can be attained by first converting the direct sequence modulation signal to an analog signal and processing that analog signal with a prior art suppressed clock pulse-duration modulation circuit might be improved to some degree at the expense of increased circuit complexity, such a trade-off generally is not satisfactory. Moreover, in such an arrangement, the maximum rate at which the circuit can be clocked is determined by the operating capabilities of the ramp generator and the digital-to-analog converter. In many cases, the maximum rate at which such a circuit can be clocked will not provide the desired degree of spectrum-spreading.
Thus, a need exists for suppressed clock pulse-duration modulation techniques and circuitry that can be employed with digital direct sequence modulation spectrum-spreading communication systems to thereby eliminate the clock information from the transmitted signal and thus further reduce the probability that the transmitted signal will be detected or intercepted by electronic surveillance receivers and/or other electronic countermeasures equipment.